In the process of semiconductor IC manufacturing, one of the final steps is burn-in. In a burn-in process, IC devices are loaded into sockets on boards which are provided with electrical contacts which provide input to the devices and test the response of the device while the board is held at elevated temperature in an oven for a prolonged period of time. In order to optimize this process, many devices are loaded into each burn-in board (BIB), and many BIBs are loaded into each oven. Prior to burn-in, the devices are located in trays for transportation through the manufacturing process. For burn-in to proceed, it is necessary for the devices to be removed from the tray and loaded into the BIB (and vice versa at the end of burn-in), the apparatus used to accomplish this being known as a burn-in board loader/unloader, one example of which is the BLU 300 available from Schlumberger Technologies, Inc., ATE Division, San Jose, Calif. The BLU 300 is described in detail in U.S. application Ser. No. 08/664,099, the contents of which are incorporated herein by reference.
In use, the BLU 300 picks a device from a location in a tray using a handler head with vacuum nozzle. The handler head is then moved to a location in the BIB and the device placed in a socket, the intent being that the action of the handler head should provide the device in a correct alignment for the device to be inserted accurately in the BIB socket. The socket itself can be provided with lead-in bevels to assist with insertion of the device. When moving devices from trays to sockets, the socket lead-ins determine how precisely the device must be positioned when it is dropped into the open socket. If the trays hold the device precisely enough, in a known location, and the socket lead-ins are generous, the device may be picked up with a simple suction cup and dropped directly into the socket. If the device is loose in the tray, the device must be precised to a more tightly controlled location before dropping it into the socket. One method for precising is to drop the device into a precising station, which centers the device, and then pick it up from the more precise location and drop it into the socket. A quicker method is to suck the device up into a pocket (precisor) on the nozzle head. The pocket centers the device, which is then dropped into the socket from the precised location. For the nozzle precisor to work properly, the device must be fully seated in the pocket. If the device is very far out of position, or if the suction cup is too stiff, or if the device is oversized, it may not seat. In this case, it is desirable to either drop the device and attempt to reaquire it from a more favorable position, or to reject the device as not suitable for placement with the tooling. A sensor is required to sense whether the device is properly seated.
When the device is dropped into the socket, it may not fully seat correctly in the socket. Among the causes for misplaced devices are a bent or damaged device or socket, an out of position nozzle, and a socket which is not fully opened. It is important that misplaced device be sensed and handled correctly. Burn in boards should be fully loaded with properly seated device to maximize testing yield. Misprecised devices need to be either replaced or rejected. It is also possible for some good devices to misinsert and be damaged if a socket is allowed to close upon the misinserted device. Misinserts must be detected in order to avoid damaging device. Previously, misinserts have been detected by allowing a lightweight flag to ride upon the back surface of the device as it falls into the socket. A sensor uses a light beam to detect whether the flag is at the proper height when the device is seated (indicates proper seating) or if the flag is too high (indicates a misinserted device). Unfortunately, these flags are prone to sticking in position and becoming useless. Also, some device must free fall into the socket in order to seat properly. Misinsert flags may interfere with the seating of the device.
It is an object of the invention to provide a system which can detect misinserted or misprecised devices without the drawbacks of the prior art systems.